Material spreading device



1968 w. c. BRADSHAW ETAL 3,403,609

MATERIAL SPREADING DEVICE Filed March 1, 1966 4 Sheets-Sheet 1 /4E FIG. 6.

/4 W/LBERT c. amosmw DOLPH w RUSCHHAUPT /N VE N TORS 14%,; rim! 4 TTORNEVS 1968 w. c. BRADSHAW ETAL $303,609

MATERIAL SPREADING DEVICE Filed March 1, 1966 4 Sheets-Sheet 2 95 94 75 94 95 BE I45 Y INVENTORS g, 98 a. W/LBERT c. amosmw 9 mg 33 00m! m RUSCHHAUPT 7 My flint! 1968 w. c. BRADSHAW ETAL 3,403,609

MATERIAL SPREADING DEVICE 4 Sheets-Sheet 5 Filed March 1, 1966 SW 5 WWW w WW x {A w r 0 mm J QQ g 5 v5 g 1963 w. c. BRADSHAW ETAL MATERIAL SPREADING DEVICE W/LBERT c. BRADSHAW DOLPH m RUSCHHAUPT INVENTORS f/Vau- 3,403,609 MATERIAL SPREADING DEVICE Wilbert C. Bradshaw and Dolph W. Ruschhaupt, Fresno, Califl, assignors to California-Fresno Asphalt Co., a corporation of California Filed Mar. 1, 1966, Ser. No. 530,889 7 Claims. (CI. 9446) The present invention relates to a material spreading device for asphalt paving machines and more particularly to such a device which, in a single pass, deposits a uniform mat of asphaltic material even on inclined and undulated surfaces. Furthermore, the spreading device of the present invention provides such uniform spreading throughout the entire range of coating materials utilized by such paving machines from paint-coat thickness to one-half inch cold sheet asphalt surfacing.

After extended periods of use, the original pavement on streets, roads and the like usually develops cracks, ravels and becomes more porous, which conditions permit water to penetrate the surface, causing general disintegration of the subsurface constituents. In the past, such old, distressed pavement was resurfaced by a multistep method consisting of applying successive coats of liquid asphalt, layers of mineral aggregate and subsequently rolling each layer to provide a relatively smooth surface. Although widely employed, such method has several disadvantages in addition to the number of operations involved. The surface provided thereby is highly susceptible to raveling by separation of the aggregate from the asphalt base. Furthermore, the asphalt material tends to bleed through the surface which results in splattering of vehicles passing thereover. The surface, even at best, has a relatively short service life, usually lasting no more than two years. More recently, an asphalt paving machine has been developed with which the spreading device of the present invention is intended to be used. This machine carries a separate supply of all the necessary ingredients of the sealing and coating material customarily used in the resurfacing of roads, streets and the like. Such ingredients usually consist of an asphaltic emulsion, mineral aggregate and water which are combined on the machine in a cold state for use by the spreading device of the present invention. It is noted that the proportions of the ingredients are varied during such mixing in the machine depending upon the type and condition of pavement to be resurfaced. Such operations consist of pavement sealing, temporary wearing surfaces in stage construction, mass crack filling, and pretreatment prior to overlay. It was found, however, that the conventional spreading equipment normally employed for applying sheet asphalt was not acceptable for use with the new machine. Such spreading equipment usually requires a hot mix, provides only a narrow range of mat thickness and makes no provision for continually agitating the asphalt material during deposit, which is necessary in the cold mix utilized by the new machine. Furthermore, the prior spreaders provide a rigid strike-off blade rigidly mounted on the spreader which would be unable to conform to transverse elevations and depressions in the old surface. This becomes a particular problem when applying the relatively thin sealing coats which, with the rigid strike-off blades, results in skinning off the high areas while depositing an excess of material in the lower areas.

Therefore, it is an object of the present invention to provide an improved spreading device for an asphalt paving machine.

Another object is to provide such an improved spreading device which insures uniform deposit of material on undulated surfaces, substantially eliminating the skinned and deep areas resulting from the use of conventional equipment.

3,403,609 Patented Oct. 1, 1968 Another object is to provide a spreading device capable of applying an asphalt sealing and covering material in a wide range of thickness.

Another object is to provide a spreading device which has a strike-off blade formed of resiliently flexible material readily conformable to transverse elevations and depressions in the surface traversed.

Another object is to provide a spreading device having such a strike-off blade which has a plurality of force applying members automatically maintaining an even predetermined pressure against the blade during movement over undulated surfaces.

Another object is to provide a spreading device of improved efliciency capable of applying a mat of asphalt material of the desired thickness in a single pass.

Other objects and advantages of the present invention will subsequently become more clearly apparent upon reference to the following description and accompanying drawings.

In the drawings:

FIG. 1 is a side elevation of the spreading device of the present invention shown mounted in trailing relation to an asphalt paving machine only fragmentarily shown for illustrative convenience.

FIG. 2 is a top plan view of the spreading device of the present invention.

FIG. 3 is a rear elevation of the spreading device.

FIG. 4 is a diagrammatic view of the air control system for the strike-off blade of the spreading device.

FIG. 5 is a somewhat enlarged transverse vertical section through the spreading device taken on line 5--5 of FIG. 2.

FIG. 6 is a somewhat enlarged fragmentary transverse horizontal section through the guide rails of the strikeofi blade adjusting members.

Referring more particularly to the drawings, a material spreading device embodying the principles of the present invention is generally indicated by the reference numeral 10. As best shown in FIG. 1, the spreading device is mounted in trailing relation on the rearward end of an asphalt paving machine fragmentarily indicated by the reference numeral 11. The paving machine provides a rearwardly extended main frame 12 having an upper superstructure 14. The superstructure mounts a mixing device commonly known as a pug mill which is fragmentarily illustrated at 15 having a material discharge pipe 16 rearwardly angularly downwardly extended therefrom. The pipe terminates in a lower discharge end 17 spaced above a support surface 20.

A pair of transversely spaced upper boom arms 22 are mounted on the superstructure 14 of the main frame 12 in rearwardly extended relation therefrom to terminate in outer ends 23. The boom arms are further supported by angular braces 25 individually connected to the boom arms adjacent to their outer ends with the opposite ends of the braces being connected to the main frame 12 by pin and bracket assemblies 26 rigidly secured to the frame as by welding or the like. A pair of elongated hydraulic lift jacks 30 provide cylinder ends 32 pivotally individually mounted on the outer ends 23 of the boom arms 22. The lift jacks include lower rod ends 34 for connection to the spreading device 10 in a manner subsequently to be described.

The paving machine 11 and the spreading device 10 of the present invention are further connected by a lift frame 40 having opposite forward and rearward plates 42 and 43, respectively, interconnected by a plurality of braces 44. The rearward plate 43 mounts a spreader box mount ing head 46 forming between it and the plate 43 upper and lower slots 47. The forward plate 42 provides sets of upper and lower brackets 50 and 52, respectively. The

upper brackets pivotally mount the cylinder end 53 of a pair of hydraulic jacks 54 having rod ends 55 pivotally connected to the main frame 12 of the paving machine 11. Rigid links 56 are pivotally connected at one of their ends to the lower brackets 52 and at their opposite ends are pivotally mounted on depending brackets 58 on the main frame to form a parallelogram tiltable connection between the lift frame 40 and the main frame 12. A pair of brackets 59 are extended from the forward plate 42 for individual connection to the rod ends 34 of the lift jacks 30.

The material spreading device 10 provides a forwardly disposed substantially upright mounting frame 60 fabricated from a plurality of transversely equally spaced upright angle bars 62 interconnected by a lower horizontal angle bar 64. The frame further includes a pair of mounting angle bars 65 having outer leg portions 66 disposed in spaced, facing aligned relation for endward sliding movement into the slots 47 in the mounting head 46 of the connecting frame 40.

An elongated spreader box 70 provides a front wall 72 rigidly connected, as by welding or the like, to the spreader mounting frame 60. The spreader box includes a rearward wall 73 disposed in spaced substantially parallel relation to the front wall which are interconnected by opposite end walls 75. The walls of the spreader box are supported in such relationship by a continuous up wardly disposed angle iron 76 rigidly connected, as by welding or the like, along the top edge of the walls and by a plurality of cross braces 78 aligned with the upright angle bars 62 of the spreader mounting frame 60. The front and rear walls 72 and 73 are sectioned and divided at the cross braces 78 so that the spreader box can be lengthened by adding additional sections or shortened by their removal, as desired. The walls of the spreader box circumscribe an elongated compartment 80 which is open at the top to receive the lower end 17 of the discharge pipe 16. The end walls 75 provide integral elongated shoe or runner portions 82 disposed in depending relation from the lower portion of the spreader box in sliding engagement with the support surface 20. The spreader box provides an open llower portion 83 so that material contained is rested upon the support surface 20.

A plurality of bearing brackets 85 are mounted in the compartment 80 of the spreader box below and in alignment with the cross braces 78 by integral web members 87 rigidly mounted on the inner surface front and rear walls 72 and 73. A pair of elongated oppositely wound screw augers 90 are journaled in bearings 92 secured in the bearing brackets 85 and provide opposite ends 94 journaled in bearings 95 in the opposite end walls 75. As best shown in FIG. 1, the screw augers provide shaft portions 97 outwardly extended from the end wall which mount sprockets 98 connected in driving relation by an endless chain 100. A larger diameter drive sprocket 102 is mounted on the rearwardmost auger extension in coplanar relation with a powered drive sprocket 104 which are interconnected by an endless chain 106. An hydraulic motor 108 is mounted on the top of the spreader box 70 for continuously rotating the drive sprocket 104 and the screw augers 90 for agitating material in the compartment 80 of the spreader box.

In order to prevent the escape of material forwardly from the spreader box beneath the front wall 72, an elongated sealing blade or apron 110 of a suitable resiliently flexible material such as composition rubber or the like is mounted in depending relation from the front wall by a plurality of bolt and nut assemblies 111 extended through the upper portion of the blade and through an elongated angle bracket 112 secured, as by welding, to the lower angle bar 64 of the spreader box. The sealing blade is of a cross sectional dimension longer than the distance between the lower edge of the front wall and the support surface 20 so as to flex rearwardly during forward movement of the paving machine in wiping relation against the support surface. A rearwardly disposed strikeotf or squeegee blade 115 of a resiliently flexible material similar to the front sealing blade 110 is mounted in a similar manner on the lower portion of the rear wall 73 of the spreader box 70 by a plurality of bolt and nut assemblies 116 extended through an elongated angle bracket 118 secured to the lower portion of the rear wall. The strike-off blade provides a lower portion 120 flexed in wiping relation against the support surface 20 when the spreader box is empty and upon material discharged therefrom when filled. In such position, the strike-off blade provides an upwardly presented surface 122 and an opposite lower surface 123 in substantially parallel relation to the support surface 20.

A pair of upstanding angle irons 126 are mounted on the rear wall 73 of the spreader box 70 individually adjacent to the opposite end walls 75 for supporting an elongated operator hand-gripping rod 127 between the upper ends of the angle irons. A plurality of angle brackets 128 are rigidly mounted on the rear wall in vertically spaced substantially parallel relation over which is mounted an elongated plate 130. A plurality of angle brackets 132 are mounted on the plate 130 in transversely equally spaced relation and in individually aligned relation with the cross braces 78 separating the sections of the spreader box 70. Each of the angle brackets 132 mounts an elongated substantially rectangular upright plate 134 thereon, as by welding or the like, which has an upper edge disposed in substantially coplanar relation with the top of the spreader box.

Each of the plates 134 mounts a series of three vertically spaced rollers 135 thereon which are individually freely rotatable about their respective mounting shaft and nut assemblies 136. A plurality of channular guide members are individually disposed in substantially upright position in surrounding relation to their respec tive sets of rollers 135 and provide oppositely spaced substantially parallel rods 142 having inner peripheries 143 disposed in contacting relation with the periphery of the rollers. The lower end of each of the guide members provides a bolted connecting flange 145 which has a lower depending rod portion terminating in a bearing 148. An elongated hinge bolt is extended through the bearing 148 for a purpose subsequently to be described.

A plurality of cylindrical force applying members are disposed in substantially transversely aligned relation upon the upper surface 122 of the strikeoff blade 115. Each of the force applying members includes a substantially upright panel 162 rigidly secured to the forwardly disposed side thereof in freely sliding relation against the plate 130 on the rear wall 73 of the spreader box 70. As best shown in FIG. 3, a pair of substantially triangular gussets .163 are mounted on the force applying members and the panels in substantially V-shaped relation alternately to mount inner and outer hinge bearings 164 and 165, respectively. The hinge bearings are disposed in aligned relation with the bearing 148 on the guide members 140 to permit relative raising and lowering of the individual force applying members about the hinge bolts 150. It is noted that the adjacent edges of each of the force applying members and panels are tapered to provide sufficient clearance for accommodating such relative angular movement of the adjacent force applying members.

Movement of the force applying members 160 is controlled by a plurality of pneumatic jacks individually associated therewith. The pneumatic jacks provide an upwardly disposed cylinder end 172 which is trunnion mounted on the rear wall 73 of the spreader box 70 by a U-shaped bracket 173 secured to the wall. The jacks include opposite piston rods ends 174 which are individually pivotally connected to their respective force applying members by a universal mounting bracket 175 disposed intermediate the ends of each force applying member. An operator support platform 176 is mounted in suspended relation from a plurality of brackets 177 rearwardly extended from the U-shaped cylinder mounting brackets 172.

As best shown in FIG. 4, the pneumatic system for supplying air under pressure to the jacks is shown providing an air compressor 180 having a pressure conduit 182 connected to a three-way pressure directing valve 184. A primary pressure conduit .185 is extended from the three-way valve for connection through a plurality of branch conduits 186 to the cylinder end 172 of the air jacks 170. The primary pressure conduit has a regulating valve 187 therein for adjusting the air pressure to the jacks and consequently the down pressure exerted against the strike-off blade 115 by the force applying members 160. An auxiliary pressure conduit 190* is extended from the three-way valve for connection to the rod ends 174 of the air jacks r170 by a plurality of branch conduits 192. Each of the branch conduits 192 has a regulating valve 193 therein for individually adjusting the pressure on the rod ends of the jacks.

Operation The operation of the described embodiment of the subject invention is believed to be clearly apparent and is briefiy summarized at this point. During transport of the asphalt paving machine to the work area, the rod end 34 of the lift jack 30 is disposed in retracted posi tion to elevate the material spreading device to a carrying position, not shown. It is noted that during actuation of the lift jack, the spreading device is raised and lowered in a substantially horizontal attitude by virtue of the parallelogram connection formed by the hydraulic jacks 54 and links 56. Upon reaching the work area, the lift jack is extended to lower the spreading device to the position shown in FIGS. 1 and 5 with the shoes 82 rested in sliding engagement upon the support surface 20'. In order to insure precise engagement of the shoes with the support surface, the tilt jacks 54 are actuated to adjust the attitude of the spreading device.

The paving machine is motivated in a forward direction and the various previously described ingredients of the asphalt material mixed in the plug mill and discharged through the discharge pipe 16. Such material is received within the compartment 80 of the spreader box 70* and upon the support surface immediately beneath the compartment. The screw augers 90 are continuously driven by the motor 108 through the sprockets 98, 102 and 104 and chains 100 and 106. Since the augers are provided with opposite spiral leads, they move the material within the compartment in opposite directions, thereby continuously to agitate and prevent separation of the various ingredients. Operation of the screw augers further maintains an even distribution of the material Within the compartment of the spreader box so that the spreading device can be operated on transversely inclined surfaces encountered on super-elevated curved roadways and the like without the material gravitating to the lower side of the compartment.

As previously described, the front sealing blade 110 is disposed in wiping relation against the support surface to prevent the leakage of asphalt material forwardly from the spreader box 70. As a result, during forward movement of the asphalt paving machine and the spreading device 10, the material within the spreader box is discharged rearwardly there-from beneath the lower surface 123 of the strike-off blade 115. The thickness of the mat spread by the device is accurately controlled by adjusting the down pressure exerted against the strikeoff blade by the force applying members 160'. Such pressure is regulated by preadjusting the regulating valve 187 to set the pressure directed to the cylinder ends 172 of the air jacks 170. It is noted that with the individual regulating valves 193 all identically positioned, an even pressure is exerted against the strike-off blade across the entire width of the spreading device.

If, during forward movement of the spreading device, a raised area is encountered anywhere along the strikeoff blade 115, the affected portion of the blade will transmit an increased pressure to its associated air jack 170. Due to the compressibility of the air in the air jack, the force applying member and strike-off blade move upwardly over such raised area by pivotal movement of the hinge bearings about the hinge bolt 150. During such upward movement, a substantially uniform downward pressure is exerted by the raised air jack 170 since the slight increase in pressure is spread to all the jacks to insure that the same depth of material is deposited on the raised area passing under the strike-off blade. Similarly, when the blade encounters a depression in the support surface, the affected portion of the strike-off blade will be actuated downwardly by the reduction in resistance against the associated jack. Accordingly, the strike-off blade conforms to the depression in the support surface and maintains the same thickness of material thereon as during deposit on a level surface.

It is noted that the force applying members may be individually raised and lowered by actuation of their associated jacks upon manipulation of the corresponding regulating valves 193. This is accomplished by manipulation of the three-way control valve to a position directing high pressure air to both conduits and 190. An imbalance between the opposite sides of a selected jack is then accomplished by manipulation of its associated regulating valve 193 to either raise or lower the selected jack with respect to the other jacks. This may be desired if a different thickness of mat is required in one portion of the spread. During such movement of the selected jack, the connecting hinges between adjacent force applying members are guided for substantially free vertical movement by the rollers 135 and guide members 140. The point contact between the rollers and the guides presents a minimum of resistance to such vertical movement and provides channels therebetween through which material may readily pass without jamming. After completing the work surface 20, the force applying members 160 are simultaneously returned upwardly to a retracted position by retraction of the air jacks 170. This is accomplished by manipulation of the three-way valve 184 individually to pressurize the auxiliary conduit and branch conduits 192. The spreading device 10 is then easily raised to a carrying position by retraction of the lift jack 30 for movement of the asphalt paving machine to the next work area.

In view of the foregoing, it is readily apparent that the structure of the present invention provides an improved spreading device for asphalt paving machines which, in a single pass, insures uniform spreading of the material irrespective of the configuration of the Surface beneath the strike-off blade. The material distributing augers permit super-elevated curves to be surfaced and sealed without starving the upper side of the curve while the automatic air controlled strike-off blade accurately conforms to elevations and depressions in the surface so that the same thickness of material is applied to any raised or depressed areas encountered in the work surface. Such uniform deposit of material is possible throughout the entire range of sealing materials from paint-coat thickness to mats of one-half inch of cold sheet asphalt surfacing.

Although the invention has been herein shown and described in What is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A material spreading device, adapted for movement upon a support surface in a predetermined forward direction of travel, comprising an elongated spreader box providing spaced forward, rearward and opposite end walls defining a material compartment having an elongated discharge opening extended transversely of said direction of travel in material depositing relation to said support surface; an elongated flexible strike-oflf blade downwardly extended from the spreader box longitudinally in coextensive relation with said opening and transversely in arcuately rearwardly flexed wiping relation against material deposited upon the support surface; and adjustable force applying means including a plurality of pivotally interconnected elongated members, vertical guide means individually mounting said elongated members in successively longitudinally substantially aligned relation along the spreader box for engagement with said strike-off blade, said members having transversely arcuate surfaces engaged against the arcuate rearwardly flexed blade; and force exerting means connected to said elongated members compressibly to urge the blade toward the support surface to control the depth of said mixture deposited on the support surface and to maintain such depth uniformly completely longitudinally of the spreader box in conforming relation to elevations and depressions encountered in the support surface.

2. The material spreading device of claim 1 including a pair of mixture agitating members providing power driven contra-rotating spiral screw augers having opposite ends journaled in said end walls of the spreader box in longitudinally extended spaced substantially parallel relation within said compartment therein, and said force exerting means providing a plurality of members mounted on said rearward wall of the spreader box and individually connected to said elongated members for their simultaneous elevational movement and being individually extendible and retractable incident to said strike-off blade encountering depressions and elevations in the support surface.

3. A strike-elf blade for paving materials and the like comprising an elongated transversely flexible member having substantially parallel opposite side edges, means connected to one of said edges mounting said edge in a substantially horizontal position for travel of said member upon a support surface in a direction transversely of its length, said flexible member extending transversely downwardly from the mounted edge thereof and thence flexing transversely arcuately rearwardly and providing substantially horizontal upwardly and downwardly disposed surfaces adjacent to said opposite edge, a plurality of independent pivotally interconnected force applying elements engaged with the upper surface of said member adjacent to said opposite edge, and force exerting means individual to said elements urging their respective elements downwardly against said member to depress the same.

4. The strike-off blade of claim 3 in which said force exerting means are individually pivotally connected to said depressing elements substantially midway between their ends to permit limited rocking movement of the depressing elements during said vertical and pivotal movement about their ends.

5. The apparatus of claim 3 including control means operatively interconnecting said force exerting means, and vertical guide means permitting individual raising and lowering of said force applying elements incident to the member encountering elevations and depressions in the support surface during said travel with said force exerting means maintaining said downward force substantially uniformly along the entire extent of the member.

6. The apparatus of claim 5 wherein said control means comprises a plurality of manually controlled elements individually operatively associated with said force exerting means for individually adjusting the downward force of said elements against said member.

7. A material spreading device comprising a spreader box having a downwardly disposed elongated discharge opening, means mounting the box for travel transversely of said discharge opening over a support surface to d posit material thereon, an elongated transversely flexible strike-off blade mounted in a transversely substantially erect attitude rearwardly adjacent to the discharge opening and extended downwardly therefrom and coextensively longitudinally thereof, said blade being adapted to flex rearwardly in dragging engagement with the deposited material so as to present an upwardly disposed substantially horizontal surface, a plurality of independent force applying elements having opposite ends interconnected for vertical and pivotal movement and engaged with the upwardly disposed surface of the blade in substantial alignment longitudinally of the blade, a plurality of jacks individually connected to said elements, and means interconnecting said jacks including a plurality of individually controlled valves disposed in respectively individual controlling relation to the jacks.

References Cited UNITED STATES PATENTS 1,709,711 4/ 1929 Duify 9444 X 1,750,896 3/1930 Lichtenberg. 1,914,950 6/1933 Kanen 9444 2,252,717 6/1941 Lundbye 9444 JACOB L. NACKENOFF, Primary Examiner. 

1. A MATERIAL SPREADING DEVICE, ADAPTED FOR MOVEMENT UPON A SUPPORT SURFACE IN A PREDETERMINED FORWARD DIRECTION OF TRAVEL, COMPRISING AN ELONGATED SPREADER BOX PROVIDING SPACED FORWARD, REARWARD AND OPPOSITE END WALLS DEFINING A MATERIAL COMPARTMENT HAVING AN ELONGATED DISCHARGE OPENING EXTENDED TRANSVERSELY OF SAID DIRECTION OF TRAVEL IN MATERIAL DEPOSITING RELATION TO SAID SUPPORT SURFACE; AN ELONGATED FLEXIBLE STRIKE-OFF BLADE DOWNWARDLY EXTENDED FROM THE SPREADER BOX LONGITUDINALLY IN COEXTENSIVE RELATION WITH SAID OPENING AND TRANSVERSELY IN ARCUATELY REARWARDLY FLEXED WIPING RELATION AGAINST MATERIAL DEPOSITED UPON SUPPORT SURFACE; AND ADJUSTABLE FORCE APPLYING MEANS INCLUDING A PLURALITY OF PIVOTALLY INTERCONNECTED ELONGATED MEMBERS, VERTICAL GUIDE MEANS INDIVIDUALLY MOUNTING SAID ELONGATED MEMBERS IN SUCCESSIVELY LONGITUDINALLY SUBSTANTIALLY ALIGNED RELATION ALONG THE SPREADER BOX FOR ENGAGEMENT WITH SAID STRIKE-OFF BLADE, SAID MEMBERS HAVING TRANSVERSELY ARCUATE SURFACES ENGATED AGAINST THE ARCUATE REARWARDLY FLEXED BLADE; AND FORCE EXERTING MEANS CONNECTED TO SAID ELONGATED MEMBERS COMPRESSIBLY TO URGE THE BLADE TOWARD THE SUPPORT SURFACE TO CONTROL THE DEPTH OF SAID MIXTURE DEPOSITED ON THE SUPPORT SURFACE AND TO MAINTAIN SUCH DEPTH UNIFORMLY COMPLETELY LONGITUDINALLY OF THE SPREADER BOX IN COMFORMING RELATION TO ELEVATIONS AND DEPRESSIONS ENCOUNTERED IN THE SUPPORT SURFACE. 